1. Field of the Invention
The invention relates to the field of apparatus and methods for the stabilization of the operation of a laser or another optical device.
2. Description of Related Art
Pulsed, Q-switched solid-state lasers are an almost ubiquitous light source for powerful, short laser pulses, as used in industry and research labs. Typically, a simple, free-running cavity design is employed, while more demanding applications require seeded lasers. In the latter case, a narrow-bandwidth, continuous wave (CW) laser beam is introduced into the cavity of the host laser. The wavelength of the CW laser is adjusted to coincide with the fluorescence maximum of the gain material of the host. When the CW laser is resonant with one of the cavity modes of the host, this mode will win the mode competition for the population inversion in the gain material with regard to the other longitudinal modes present in the free running host laser.
When the host laser is seeded, the bandwidth produced is reduced dramatically. In the case of an un-seeded Nd:YAG laser, the width of the fluorescence maximum at 1064 nm is ˜20 GHz, while the bandwidth of a seeded Nd:YAG laser with a pulse duration of 8 ns is typically 0.1 GHz, a factor of 200 smaller than that of the un-seeded laser. The narrow bandwidth is required for applications in spectroscopy, and for pumping narrow-bandwidth optical parametric oscillators (OPO). Similarly, the coherence length of these light pulses increases from ˜1.5 cm to 3 meters, which is important for coherent detection schemes, such as coherent Laser Imaging Detection and Ranging (Lidar), and Coherent Anti-Stokes Raman Scattering (CARS).
The seeded lasers also show superior pulse characteristics. In free-running, Q-switched lasers, each pulse is modulated by beating between the longitudinal modes generated in the cavity. Because of the random nature of these modes, each consecutive pulse shows a different shape. The free running modes are built up from the vacuum background. The time required to build up a mode is also subject to random behavior, causing jitter in the timing of the generated pulse.
In seeded lasers the pulse is built up from the injected radiation, eliminating the random behavior. The generated radiation only contains one longitudinal mode and, as a result, no beating artifacts.
Seeded Nd:yag lasers are currently available from a number of suppliers, such as Continuum (coherent), Spectra physics, and Quantel. Most of these are expensive mainframe lasers, e.g. more than $100,000 in 2006. All these lasers are based on the same design as shown in FIGS. 1a and 1b. In these lasers, the Q-switch is formed by a polarizer 14 and a Pockel's cell 30 placed in front of the back reflector 12 of the cavity. The seed-beam is brought into the cavity via the polarizer with the same polarization as the reflected beam. The electronic speed of the Pockel's cell 30 is reduced, increasing the build-up time of the pulse in the oscillator. The timing of the generated pulse is then observed with a photo diode. The rear mirror 12 in the lasers is mounted in a piezo-electric element 32, and its position is dithered. A lock-in scheme then allows minimizing the build-up time of the oscillator, making the cavity resonant with the seed-beam. The cavity of the host laser is also fitted with two quarter-wave plates, so that the beams traveling either way through the cavity have opposite circular polarization. In this way, the generation of a second longitudinal mode through spatial hole burning is prevented.